1. Field of the Invention
The present invention relates to a signal processing apparatus and method for use in a magnetic resonance imaging (MRI) system, and more particularly to such an apparatus and method which increases the signal to noise ratio of an MRI signal.
2. Discussion of the Related Art
As is well known, MRI signals are generated when an object to be imaged is located in a magnetic field. The magnetic field causes the magnetic dipole moment of each proton (or other nucleus) in the object to precess at a specific frequency, often called the Larmor frequency. The Larmor frequency of a specific proton in the object is proportional to the local strength of the magnetic field at the position of that proton.
Conventional MRI devices establish magnetic fields that vary with respect to location and time. Therefore, information regarding various locations within the object to be imaged can be associated with a known frequency or range of frequencies and with a known time or period of time.
Conventionally, MRI devices are known that use multi-coil sensing systems to sense the MRI signal induced by the precessing nuclei. For example, multi-coil systems include two or more coils arranged in a specific spatial relationship to provide good signal strength for a desired imaging application. Commonly, several coils are provided in a planar relationship for use in sagittal scanning of the human back.
When using a multi-coil system, a magnetic field gradient can be established such that each coil of the system may be generally associated with a given frequency range. Adjacent coils usually have adjacent frequency ranges. For example, in a sagittal scanning system, a magnetic field may be generated that is weak at the base of the spine of the person being imaged and that increases in strength towards the head of the person. Often, a constant magnetic field gradient is used which creates a frequency gradient, expressed in hertz per centimeter, in the area in which the object to be examined is located.
Therefore, in a sagittal scan of the human back, for example, each section of the back may be generally associated with a given frequency range, and the imaging signal information along the length of the back ca be received by the multi-coil system. However, due to the nature of the coil responses as a function of position of the precessing protons (or nuclei), it should be understood that some overlap between the frequency sensitivities of the various coils in the multi-coil system will exist.
In conventional multi-coil systems, the output from each coil is summed to produce an output signal which includes image information for the entire imaging field. A drawback in this conventional summing technique is that noise generated by each coil in the multi-coil system is also summed. Therefore, the signal output from the summing device includes a high level of noise and the image quality is accordingly degraded.
A source of this noise is "Johnson noise," which is the noise created when an electrical circuit includes a resistive component, at temperatures above absolute zero. For example, when imaging human tissue, small induced currents are often generated within the slightly conductive human tissue by the time varying magnetic fields induced by the current through the MRI sensing coil. Thermal agitation of electrons in these resistive paths generates random voltage fluctuations (i.e. Johnson noise) that degrade the MRI signal. Johnson noise is broadband and theoretically exhibits a flat spectrum of noise power per unit bandwidth, as a function of frequency.